Phosphatic wastes which are produced during the mining and beneficiation of phosphate ore usually are disposed of by pumping into storage ponds. For example, phosphoric acid produced by the dihydrate, hemihydrate or anhydrate process produces a clay waste frequently referred to as phosphoric acid slime in the washing and beneficiation operation. It is well recognized that the natural settling rate of phosphatic clay waste and phosphoric acid slimes is extremely slow. The suspended solids in such wastes and slimes have a rapid initial settling rate and will frequently settle from an initial value of about 3% suspended solids to about 5 or 6% suspended solids by the end of the first day. At the end of the first week such wastes and slimes frequently settle to about 10% suspended solids. Frequently, after about three months the solid content of the waste and slimes increases to about 15%. Further densification, however, is extremely slow. The time required to settle to about 18% suspended solids usually requires about one year. Concentrations to 20% or higher suspended solids usually requires several years. Sedimentation beyond 20% suspended solids is influenced by the individual characteristics of the particular slime which is related to the characteristics of the ore, and the particular pond maintenance procedures employed.
The magnitude of the water problem in phosphate rock production can be readily appreciated when it is realized that between 20,000 and 30,000 gallons of water per minute are utilized in a typical phosphate mine. As discussed above, to accommodate such clay waste and slimes, large impoundments covering 400 to 600 acres, with dam heights ranging from 20 to 40 feet are frequently employed. When these impoundments or ponds become filled with such wastes or slimes, new ponds must be built if phosphate rock production is to continue. Both completely filled ponds and operating ponds present potential environmental hazards due to possible dam failure. Reclaiming of impoundment areas generally is not possible before decades have passed after their filling. Even then, reclaiming such ponds for agricultural and other uses remains a difficult and costly procedure.
Thus, in the normal course of mining and washing operations, as active waste and slime ponds become filled and hence inactive, there is created a number of abandoned areas in such disposal acreage which may be as much as 40 years old or older. Generally, even these long abandoned areas will not have settled or dried to a high solids content. It has been reported, in fact, that there is little change in the solids content of old abandoned ponds with time.
FIG. 1 is an example of a typical settling rate curve for phosphatic slimes which is plotted on a semi-log scale. Although the initial settling rate over the first 10 to 12 months is fast enough to allow for recovery of about 80 to about 85% of the water contained in the waste or slimes pumped into the settling pond, approximately 10 to 20% of the water is lost by the combination of evaporation and confinement in the settled slimes. It has been estimated that this is equivalent to about 5 to 7 tons of water for each ton of phosphate rock produced.
FIG. 1 also shows that the settling rate for such aqueous slime mixtures exhibits a relatively steep slope at first, followed by a gradual flattening as hindered settling of the colloidal particles becomes more predominant. This, in turn, causes the solids concentration of the settled material to increase very slowly with time. This is due to the fact that such suspended solid particles settle relatively fast in a dilute pulp, but as the particles in the slime mixture become crowded closer together, the settling rate slows to a point where for all practical purposes an ultimate or limiting pulp density is reached.
There also have been studies on processes developed for the dewatering phosphatic wastes and slimes by treating such materials with a flocculant such as polyethylene oxide (PEO) and dewatering the resultant flocs with mechanical devices such as rotary screens. Although these processes are effective for accelerating the dewatering of such phosphatic waste and slimes, they suffer from the disadvantage of requiring a relatively expensive flocculant, the major part of which cannot be recovered and is forever lost with the agglomerated slimes. Furthermore, the water that is recovered from such flocculant processes is subjected to contamination by the flocculant and as a result thereof may not be suitable for reuse elsewhere in the mining and beneficiation operation without the separation and removal of the flocculant. For example, flocculant contaminated water may not be suitable for the flotation process because the flocculant can effect the flotation agents adversely. Furthermore, it is known that although flocculants will cause agglomeration with some clays of some phosphate rock, that flocculants will not cause effective and/or economical agglomeration with the clays of all phosphate rock.
Even where flocculants are capable of causing slimes to agglomerate, the process still requires large trommel screens to separate the agglomerated slimes from the separated water. Even then the water may be still highly contaminated with precipitated solids which have not been separated from the water due to the relatively poor separation achieved by such flocculant induced agglomeration processes. Furthermore, processes using flocculants and trommel screens cannot concentrate slimes more than about 20% solids concentration.
Crossflow filtration has been used for processes such as reverse-osmosis, dialysis, and ultra-filtration. In general, these particular processes treat relatively and usually extremely dilute solutions and/or suspensions. Crossflow filters for such processes have used both tubular and spiral-wound membranes.
U.S. Pat. No. 4,301,013 discloses a spiral-wound membrane which is particularly useful for the concentration of cheese whey in the dairy industry. U.S. Pat. No. 4,299,702 discloses another spiral-wound type membrane useful in reverse osmosis or ultrafiltration separation processes. U.S. Pat. No. 3,401,798 discloses another spirally-configured, semi-permeable membrane laminate apparatus for separating constituants in a feed slurry, wherein permeable membranes are disposed on opposite surfaces of a laminate In commercial product bulletins, tubular membrane configurations and spiral-wound membrane configurations are disclosed as useful separation devices in the electrocoat paint and dairy industries and oil recovery, oil/water separation, protein recovery, enzyme concentration, latex and PVA concentration and cationic electrophoretic paint recovery processes. Reported preliminary tests of concentration of phosphate slimes by crossflow filtration, using woven fire-hose jackets and 325-mesh stainless steel screen supports, apparently in a tubular configuration, resulted in 47% water recovery at a flow rate ostensibly less than 5 ft/sec.
Accordingly, it can be appreciated that there is a need for new technology and processes to reduce or greatly eliminate the need for settling ponds for phosphatic waste and phosphoric acid slimes. The elimination or substantial reduction of such impoundment areas not only permits the land to be used for other purposes and eliminates the potential hazard of dam failure, but also substantially reduces the time the major part of the process solvent, usually water, is in inventory in the slimes treatment system. Furthermore, slimes which have been densified to about 27% to about 30% or higher by weight suspended solids have a sufficiently high enough density, and small enough volume, to enable such wastes and slimes to be completely disposed of in the mined out cavity from which their derivative phosphate rock was removed.